The memorable broadcast from February 1968 which chronicled Walter Cronkite's visit to the front lines of Vietnam following the Tet Offensive.
The special ended with the now legendary personal commentary from Cronkite declaring that the war was unwinnable, and that the best option was to negotiate for an end to the battle. That analysis would famously lead Lyndon Johnson, watching the broadcast, to declare "If I've lost Cronkite, I've lost middle America."
My thanks to the late YouTube channel btm0815ma for first providing this nugget.